The role of the conceptus in determining litter size

Abstract

The Chinese Meishan pig gives birth to three to five more pigs per litter than lessprolific U.S. or European pig breeds as a result of a markedly decreased placental size and an increased piglet weight:placental weight ratio (placental efficiency). We hypothesized that as a result of their intense selection for prolificacy, the Chinese had indirectly selected for a smaller, more efficient placenta in the Meishan breed. The goals of this study were to determine whether 1) significant variation in placental size and efficiency existed within our population of purebred Yorkshire pigs and 2) selection of piglets (boars and gilts) based on ^ Journal paper number J-17947 of the Iowa Agricultural and Home Economics Experiment Station, Ames, Projects 3505 and 0200, and supported in part by Hatch Act and State of Iowa Funds. Reprinted with permission of the American Society of Animal Science firom the Journal of Animal Science 77:1654-1658 (1999) 53 clear differences in placental size and efficiency would affect litter size. We found that there was significant (approximately threefold) variation in placental efficiency in our herd of Yorkshire pigs, with marked (approximately twofold) variation existing within individual litters. We then selected piglets (boars and gilts) that had either a higher (A Group) or lower (B Group) than average placental efficiency. Although the birth weights of selected A Group piglets were similar to those of the B Group piglets, they had markedly smaller placentae. We bred males from each group (A or B) to the females of the same group and collected farrowing data fi^om parities 1 and 2. In both parities, A Group females farrowed more live piglets per litter than did B Group females (12.5 ± .7 vs 9.6 ± .5, P < .05). Although A Group piglets were on average approximately 20% lighter than B group piglets (1.2 ± .1 vs 1.5 ± .1 kg, P < .05), their placentae were approximately 40% lighter (250 ± 10 vs 347 ± 15 g, P < .01), resulting in a marked increase in placental efficiency. The results of this study suggest that selection on placental size and efficiency may provide a valuable tool for optimizing litter size in commercially important pig breeds.